1. Field of the Invention
The present invention relates generally to a device for mounting an optical moisture sensor upon the interior surface of a windshield, and more particularly, to a mounting device which installs the optical moisture sensor in a vacuum to eliminate air pockets when tape mounting the sensor to the surface of the windshield.
2. Summary of Related Art
Motor vehicles have long been equipped with motor-driven windshield wipers for clearing moisture from the external surface of the windshield, at least within the driver's field of vision, and generally over a larger area so as to enhance vision through the windshield. In most vehicles today, the windshield wiper system includes multi-position or variable speed switches which allow the driver to select a wide, if not an infinitely variable, range of speeds to suit conditions. Wiper controls are manually operated and typically include a delay feature whereby the wipers operate intermittently at selected time delay intervals.
Wiper control systems have recently been developed which include a moisture sensor mounted on the windshield to automatically activate the motor when moisture is deposited upon the surface of the windshield or other vehicle window upon which a wiper may be employed, such as the rear window.
McCumber et al. (U.S. Pat. No. 4,620,141) disclose an automatic control circuit for triggering a sweep of the wiper blades in response to the presence of water droplets on the exterior surface of a windshield. A block-like sensor housing is mounted upon the interior surface of the windshield. The construction of the sensor and its associated circuitry are fully described in the patent, and the disclosure is incorporated herein by reference.
A number of the sensing or detecting units for automatic wiper activation operate upon the principle of a light beam being diffused or deflected from its normal path by the presence of moisture on the exterior surface of the windshield. The systems which employ optical sensors have the singular advantage that the means of sensing (i.e. disturbances in an optical path) is directly related to the phenomena observed by the driver (i.e., disturbances in the optical path that affords the driver vision).
The rain sensor systems for controlling the windshield wipers of a vehicle as disclosed by McCumber et al. and Teder (U.S. Pat. No. 5,059,877) include a box-like housing mounted upon the interior surface of the windshield. The presence of moisture on the surface of the windshield affects the reflection of light at the air-glass interface, and this change in reflected light is electronically processed and utilized as the signal for activating the windshield wipers.
In order for the system to operate properly the sensor housing must remain in a fixed position relative to the windshield surface, and the light pipes or rods must be optically coupled to the windshield so as to prevent spurious reflection of light from the interior surface of the windshield as would be caused, for example, by moisture condensation or dust accumulation on the surface. In other words, the sensor housing should securely engage the windshield and be optically coupled to the windshield so as to effectively eliminate the interface between the light pipes or rods and glass surface from an optical standpoint.
Vehicle manufacturers and windshield replacement retail outlets offer the moisture-sensing control system as an optional feature. Instead of having the moisture sensor system installed when the windshield is produced by a glass manufacturer, which would require a separate inventory for windshields with moisture sensors, the vehicle manufacturers and windshield retail outlets require a means for selectively installing the sensor system at their own facility.
Two main problems occur in connection with the selective installation of the moisture sensor system. The first problem involves the electrical interface of the moisture sensor system with existing vehicle electrical windshield wiper systems. An electrical system is disclosed in U.S. Pat. No. 5,239,244 to Teder which solves the electrical interface requirement. The control system facilitates the integration of a windshield moisture-sensitive wiper control system into an existing pulse-wipe windshield-wiper system control which utilizes the same wiring harness and is compatible with the existing system. From an electrical standpoint, the moisture sensor control system may be selectively installed on a windshield by the motor vehicle manufacturer or by the replacement windshield outlet with no adverse impact.
The second problem is mounting the sensor housing on the interior surface of the windshield to maintain the optical integrity of the moisture sensing system. U.S. Pat. No. 5,262,640 to Purvis et al. describes an intermediate adhesive interlayer for affixing the sensor housing to the windshield. The sensor housing is affixed directly to the surface of the windshield or other vehicle window by means of an intermediate interlayer disposed between the sensor housing and the interior surface of the windshield. The intermediate interlayer preferably comprises a double-faced adhesive body which adheres to both the interior surface of the window and the opposed surface of the sensor housing. The light pipes or rods of the sensor unit are optically coupled to the interlayer and the interlayer, in turn, is of such transparency as to adequately optically couple the light pipes or rods directly to the windshield or other window unit.
An additional problem area has developed in using the adhesive interlayer to secure the sensor housing. When the smooth surface of the adhesive interlayer is pressed against the flat surface of the windshield glass, air may be trapped between the adhesive layer and the glass to form an undesirable air pocket or air bubble. Pervis et al. describe optical interface regions that are less prone to the formation of air bubbles than prior art, but the techniques disclosed do not eliminate the problem entirely.
The vacuum-assisted device of the present invention provides a significant improvement in reducing the air pockets. Air bubbles in the adhesive layer are undesirable because they degrade the optical performance of the moisture sensor control system. Further, air bubbles tend to push the optical sensor away from the glass, compromising mechanical performance and prohibiting good tape contact. Finally, the inconsistent and irregular nature of air bubbles make them appear unsightly to an observer examining the outside surface of the windshield.
The problem of air bubble formation in the adhesive interlayer can be somewhat alleviated by increasing the force by which the sensor is pressed against the glass. However, the tendency of the adhesive interlayer to trap air cannot be eliminated entirely. In addition, adhesives are difficult to apply in an automotive assembly plant where the windshields are installed.
Another method for alleviating air bubble formation in the adhesive interlayer is matching the optical interface surface of the moisture sensor to the curvature of the windshield glass. However, this technique does not eliminate trapped air entirely. Matching the curvature of the sensor housing to the curvature of the windshield requires precise tolerances on the surface of the sensor housing, which would increase the cost of the device. In addition, the shape of the sensor housing would have to be designed for a single window configuration, which necessitates a different design for each windshield configuration. Consequently, it would be highly desirable to devise a means of sensor attachment which did not require great precision for mounting the sensor housing on the windshield.
In addition to air pockets, further problems may be caused by the use of excessive force in order achieve good contact of the adhesive interlayer. The moisture sensor housing must be pressed against the windshield with considerable force to ensure that the adhesive interlayer contacts over the entire surface and flows enough to accommodate imperfections. The need to apply this force leads to potential problems of fatigue and consistency in the production operations for the preparation and installation of the windshield with moisture sensor.
In addition, a force applied by hand, or with any device which may be temporarily attached to the frame of the vehicle, places an outward force on the windshield. This may damage the windshield during the installation process.
The moisture sensor control systems with optical sensors require that an interface region be optically coupled to the windshield glass. Other systems, such as described by Larson in U.S. Pat. No. 4,859,867 do not use the adhesive as an optical coupling agent and thus to not require good optical performance from the adhesive. Such systems still, however, are rendered less aesthetically pleasing by the presence of air bubbles in the adhesion of the sensor to the glass.
One means for avoiding the air bubbles and other mounting difficulties in mounting the sensor housing is to attach the moisture sensor housing to a bracket which is permanently installed on the windshield. Such a system is described by O'Farrel in U.S. Pat. No. 4,973,844. The mounting bracket is typically installed under great heat and pressure at the windshield manufacturers, in a process similar to that of installing brackets to secure rear-view mirrors. This technique, however, requires a separate inventory for windshields with brackets for securing a rain sensor. The carrying costs for the additional inventory can be significant.